Activated oxides with carriers reactions

Reactions of Activated Oxides with Carriers—Catalytic Properties... 

The PPR and LFR are also applied in a more recently developed dedicated process for NOx removal from off-gases. The Shell low-temperature NO reduction process is based on the reaction of nitrogen oxides with ammonia (reactions iv and v), catalyzed by a highly active and selective catalyst, consisting of vanadium and titania on a silica carrier [18]. The high activity of this catalyst allows the reaction of NO with ammonia (known as selective catalytic reduction) to be carried out not only at the usual temperatures around 300°C, but at substantially lower temperatures down to 130°C. The catalyst is commercially manufactured and applied in the form of spheres (S-995) or as granules (S-095) [19]. 

There are two kinds of redox interactions, in which ubiquinones can manifest their antioxidant activity the reactions with quinone and hydroquinone forms. It is assumed that the ubiquinone-ubisemiquinone pair (Figure 29.10) is an electron carrier in mitochondrial respiratory chain. There are numerous studies [235] suggesting that superoxide is formed during the one-electron oxidation of ubisemiquinones (Reaction (25)). As this reaction is a reversible one, its direction depends on one-electron reduction potentials of semiquinone and dioxygen. 

There have been many attempts to relate bulk electronic properties of semiconductor oxides with their catalytic activity. The electronic theory of catalysis of metal oxides developed by Hauffe (1966), Wolkenstein (1960) and others (Krylov, 1970) is base d on the idea that chemisorption of gases like CO and N2O on semiconductor oxides is associated with electron-transfer, which results in a change in the electron transport properties of the solid oxide. For example, during CO oxidation on ZnO a correlation between change in charge-carrier concentration and reaction rate has been found (Cohn Prater, 1966). 

A second report of organic carbonate production from epoxide and C02 utilizes copper(I) cyanoacetate, Cu(02CCH2CN), as a carrier of activated C02 (158). Reaction of propylene oxide with Cu(02CCH2CN) at 130°C for 10 hours yields propylene carbonate in 83% yield, based on the... 

The adsorption of biomolecules onto carriers that are insoluble in water is the simplest method of immobilization. An aqueous solution of the biomolecules is contacted with the active carrier material for a defined period of time. Thereafter the molecules that are not adsorbed are removed by washing. Anionic and cationic ion exchange resins, active charcoal, silica gel, clay, aluminum oxide, porous glass, and ceramics are being currently used as active material. The carrier should exhibit high affinity and capacity for the biomolecule and the latter must remain active in the adsorbed state. The carrier should adsorb neither reaction products nor inhibitors of the biocatalyst. 

Mercaptans are converted in disulfides by oxidative dehydrogenation. The reaction is carried out for sweetening hydrocarbons, such as jet fuel gasoline kerosine and gas oil. The reaction is catalysed by a mixture of transitional metals as Cu, Mn, Fe and Co with activated carbon as catalyst carrier. The process is developed by UOP under the trade name MEROX [15-17]... 

ScTiOj and LiNbOj. These compounds share many features with non-transition-metal oxides. They have a filled valence band of predominantly 02p character and a gap between the valence band and an empty conduction band. Typical band gaps are h-A eV. Unlike transition-metal oxides with 0 < n < 10, stoichiometric, post-transition-metal oxides ZnO, SnO, and transition-metal oxides may be reduced but not oxidized. The post-transition oxides ZnO, In Oj, SnOj, as well as the majority of transition-metal oxides, are active in redox reactions since the electron configuration of the solid may be altered. However, the reaction with oxidizing species such as Oj is expected only with samples that have been bulk reduced or where the surfaces have been made oxygen deficient (Calatayud et al. 2003). The reduction of post-transition oxides as a rule leads to the formation of free carriers, which greatly increase the metal-oxide conductivity, a fact that is crucial for sensor applications. 

The modern catalyst is based on a vanadium-molybdenum mixed oxide, with a variety of promoters which increase activity and selectivity. The active phase is supported on an inert carrier, which allows the efficient removal of the reaction heat. 

In the course of our work on deposition precipitation onto pre—shaped carrier particles we have found that under certain circumstances the technique can also be used for realizing controlled non—uniform activity distributions over carrier bodies [6], After careful consideration it turned out that the basic phenomenon underlying this finding is connected with transient pH gradients occurring in carrier particles directly after liquid imbibition. In case the rate of the precipitation reaction in question depends on the pH of the aqueous phase, the metal (oxide) distribution after precipitation will reflect the previous pH gradient. In this paper the principles of this new application of precipitation reactions in catalyst synthesis are elaborated. Furthermore, several examples of catalysts with their respective metal distributions are described. 

The corrosion of iron occurs particularly rapidly when an aqueous solution is present. This is because water that contains ions provides an oxidation pathway with an activation energy that is much lower than the activation energy for the direct reaction of iron with oxygen gas. As illustrated schematically in Figure 19-21. oxidation and reduction occur at different locations on the metal surface. In the absence of dissolved ions to act as charge carriers, a complete electrical circuit is missing, so the redox reaction is slow, hi contrast, when dissolved ions are present, such as in salt water and acidic water, corrosion can be quite rapid. 

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